Abstract
The Vero4DRT (MHI‐TM2000) is capable of performing X‐ray image‐based tracking (X‐ray Tracking) that directly tracks the target or fiducial markers under continuous kV X‐ray imaging. Previously, we have shown that irregular respiratory patterns increased X‐ray Tracking errors. Thus, we assumed that audio instruction, which generally improves the periodicity of respiration, should reduce tracking errors. The purpose of this study was to assess the effect of audio instruction on X‐ray Tracking errors. Anterior‐posterior abdominal skin‐surface displacements obtained from ten lung cancer patients under free breathing and simple audio instruction were used as an alternative to tumor motion in the superior‐inferior direction. First, a sequential predictive model based on the Levinson‐Durbin algorithm was created to estimate the future three‐dimensional (3D) target position under continuous kV X‐ray imaging while moving a steel ball target of 9.5 mm in diameter. After creating the predictive model, the future 3D target position was sequentially calculated from the current and past 3D target positions based on the predictive model every 70 ms under continuous kV X‐ray imaging. Simultaneously, the system controller of the Vero4DRT calculated the corresponding pan and tilt rotational angles of the gimbaled X‐ray head, which then adjusted its orientation to the target. The calculated and current rotational angles of the gimbaled X‐ray head were recorded every 5 ms. The target position measured by the laser displacement gauge was synchronously recorded every 10 msec. Total tracking system errors (ET) were compared between free breathing and audio instruction. Audio instruction significantly improved breathing regularity (p < 0.01). The mean ± standard deviation of the 95th percentile of ET (E95T) was 1.7 ± 0.5 mm (range: 1.1–2.6 mm) under free breathing (E95T,FB) and 1.9 ± 0.5 mm (range: 1.2–2.7 mm) under audio instruction (E95T,AI). E95T,AI was larger than E95T,FB for five patients; no significant difference was found between E95T,FB and ET,AI95(p = 0.21). Correlation analysis revealed that the rapid respiratory velocity significantly increased E95T. Although audio instruction improved breathing regularity, it also increased the respiratory velocity, which did not necessarily reduce tracking errors.PACS number: 87.55.ne, 87.57.N‐, 87.59.C‐,
Highlights
Respiratory motion is one of the most important issues to be addressed in radiotherapy.[1,2] Respiratory motion broadens the dose distribution in the anatomy moving near the beam edges for conventional radiotherapy with uniform radiation intensity[3] and significantly degrades the dosimetric advantage of intensity-modulated radiotherapy due to the interplay between the motion of a multileaf collimator (MLC) and respiratory motion.[4,5] These impacts can be strongly enhanced, for hypofractionated radiotherapy
Respiratory motion data Anterior–posterior (AP) abdominal skin-surface displacements obtained from ten lung cancer patients under free breathing and simple audio instruction were used as an alternative to tumor motion in the superior–inferior (SI) direction
Audio instruction mostly led to an increase in respiratory velocity, which could be a factor in preventing reduction in X-ray Tracking errors
Summary
Respiratory motion is one of the most important issues to be addressed in radiotherapy.[1,2] Respiratory motion broadens the dose distribution in the anatomy moving near the beam edges for conventional radiotherapy with uniform radiation intensity[3] and significantly degrades the dosimetric advantage of intensity-modulated radiotherapy due to the interplay between the motion of a multileaf collimator (MLC) and respiratory motion.[4,5] These impacts can be strongly enhanced, for hypofractionated radiotherapy.The American Association of Physicists in Medicine Task Group 76 has suggested several approaches to overcome the above shortcomings induced by respiratory motion, such as breath-holding, respiratory-gating, and dynamic tumor-tracking (DTT) techniques.[6]. We have developed a four-dimensional image-guided radiation therapy system with a gimbaled X-ray head, the Vero4DRT (MHI-TM2000) (Mitsubishi Heavy Industries, Ltd., Tokyo, Japan; BrainLAB, Feldkirchen, Germany)(7,8) (Fig. 1). This system has three special features: 1) an O-ring-shaped gantry, 2) a gimbaled X-ray head, and 3) orthogonal kV X-ray imaging subsystems. The Vero4DRT can separately rotate the gantry along an O-shaped guide lane and the O-ring along its vertical axis, providing noncoplanar three-dimensional (3D) conformal beam delivery without a treatment couch rotation. The gimbaled X-ray head, which comprises a compact 6 MV linear accelerator with a C-band klystron and system-specific MLC,(9) is mounted on the inside of the O-ring-shaped gantry. Two orthogonal sets of kV X-ray tubes and flat panel detectors (FPDs) with a spatial resolution of 0.2 mm at the isocenter level are mounted in the O-ring-shaped gantry to simultaneously acquire arbitrary orthogonal fluoroscopic images.[10]
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